Nitric oxide (NO) is involved in the regulation of many physiological processes as well as the pathophysiology of a number of diseases. It is synthesized enzymatically from L-arginine in numerous tissues and cell types by three distinct isoforms of the enzyme NO synthase (NOS). Two of these isoforms, endothelial NOS (eNOS) and neuronal NOS (nNOS) are expressed in a constitutive manner and are calcium/calmodulin dependent. Endothelial NOS is expressed by endothelium and other cell types and is involved in cardiovascular homeostasis. Neuronal NOS is constitutively present in both the central and peripheral nervous system where NO acts a neurotransmitter. Under normal physiological conditions, these constitutive forms of NOS generate low, transient levels of NO in response to increases in intracellular calcium concentrations. These low levels of NO act to regulate blood pressure, platelet adhesion, gastrointestinal motility, bronchomotor tone and neurotransmission.
In contrast, the third isoform of NOS, inducible NOS (iNOS), a virtually calcium independent enzyme, is absent in resting cells, but is rapidly expressed in virtually all nucleated mammalian cells in response to stimuli such as endotoxins and/or cytokines. The inducible isoform is neither stimulated by calcium nor blocked by calmodulin antagonists. It contains several tightly bound co-factors, including FMN, FAD and tetrahydrobiopterin. The inducible isoform of nitric oxide synthase (NOS2 or iNOS) is expressed in virtually all nucleated mammalian cells following exposure to inflammatory cytokines or lipopolysaccharide.
The enzyme iNOS synthase is a homodimer composed of 130 kDa subunits. Each subunit comprises an oxygenase domain and a reductase domain. Importantly, dimerization of the iNOS synthase is required for enzyme activity. If the dimerization mechanism is disrupted, the production of nitric oxide via inducible NOS enzyme is inhibited.
The presence of iNOS in macrophages and lung epithelial cells is significant. Once present, iNOS synthesizes 100-1000 times more NO than the constitutive enzymes synthesize and does so for prolonged periods. This excessive production of NO and resulting NO-derived metabolites (e.g., peroxynitrite) elicit cellular toxicity and tissue damage which contribute to the pathophysiology of a number of diseases, disorders and conditions.
Nitric oxide generated by the inducible form of NOS has also been implicated in the pathogenesis of inflammatory diseases. In experimental animals, hypotension induced by lipopolysaccharide or tumor necrosis factor alpha can be reversed by NOS inhibitors. Conditions which lead to cytokine-induced hypotension include septic shock, hemodialysis and interleukin therapy in cancer patients. An iNOS inhibitor has been shown to be effective in treating cytokine-induced hypotension, inflammatory bowel disease, cerebral ischemia, osteoarthritis, asthma and neuropathies such as diabetic neuropathy and post-herpetic neuralgia.
In addition, nitric oxide localized in high amounts in inflamed tissues has been shown to induce pain locally and to enhance central as well as peripheral stimuli. Because nitric oxide produced by an inflammatory response is thought to be synthesized by iNOS, the inhibition of iNOS dimerization produces both prophylactic and remedial analgesia in patients.
Hence, in situations where the overproduction of nitric oxide is deleterious, it would be advantageous to find a specific inhibitor of iNOS to reduce the production of NO. However, given the important physiological roles played by the constitutive NOS isoforms, it is essential that the inhibition of iNOS has the least possible effect on the activity of eNOS and nNOS.